1. Field of the Invention
The invention relates generally to the field of geodetic location of electronic devices within an enclosed space. More particularly, the invention relates to ultrasonic techniques for precise location of mobile electronic devices within the confines of a building.
2. Background Art
Determining the position (e.g., in Cartesian coordinates x, y, z) of mobile electronic devices is important in a number of technical fields, or example, global positioning systems (GPS), indoor positioning (and/or tracking) systems (IPS), context-aware (or location-aware) computing systems, augmented reality systems, ubiquitous (or pervasive) computing systems, range-finding systems, navigation systems, autonomous robotic systems, location-based services, enhanced emergency telephone “911” (E911) services, radio frequency identification systems (RFID), smart rooms and offices, and social networking.
The enabling components for position determination are the receiver (sensor or detector) and transmitter (emitter), denoted as Rx and Tx, respectively, in the present description. The operating principle underlying any particular Rx/Tx pair may be based in propagation of sound (ultrasound) or of light (visible, infrared (IR)) or radio frequency (RF)). The medium of transmission between the transmitter and receiver may be through rock (e.g. seismic imaging), water (e.g. sonar), air (e.g. radar) or vacuum (e.g. radio telescope or very long baseline interferometer). The medium characterizes the properties and speed of transmission between the transmitter and the receiver. Any particular Rx/Tx pair may have varying degrees of “intelligence” and span the range of active versus passive operation.
There are two approaches known in the art for the architecture of position determination systems, namely, client-centric and server-centric architectures. Client-centric systems typically provide on-demand delivery of location information to a client device. GPS is an example of a client-centric architecture. Server-centric systems generally are based on external monitoring devices for tracking the position of the mobile electronic device and for storing the position of the device in a database. RFID is an example of a server-centric architecture. The distinction between client-centric and server-centric architectures is important in the context of the privacy and security of persons, places and systems (privacy policy management).
The utility of the Rx/Tx pair for position determination is dependent on the algorithm or method by which the raw physical inputs are processed to yield absolute or relative position. Algorithms such as time-of-flight (range determination), triangulation, multi-lateration, tomography, and interferometry can be used to determine position(s) of one or more devices in one, two or three dimensions at an instant of time (ID, 2D, and 3D) or dynamically with time (4D). The underlying devices may combine analog and/or digital components. Digital systems will have additional complexity based on the digitization, transmission, processing, storage and value-added delivery of digital data and services.
Finally, the increasing availability of location-aware devices is occurring both on desktop computer systems for the home and office as well as mobile devices such as GPS receivers, cell phones and personal data assistants (PDAs). All such devices are increasingly linked via the Internet, which has evolved to support increasingly complex software systems ranging from the early email, remote login and file transfer protocols; the invention of the world wide web (WWW) and browser technology; and most recently the emergence of search engines, web services (Web 2.0) and service oriented architectures (SOA).
The performance of a position determination system determines to a large degree the range of its applications. System performance measures include responsiveness, accuracy, precision, and cost. A commercially acceptable location device typically occupies a niche market based on engineering trade-offs in such performance measures. Added to this are ancillary features of the device relating to calibration, operations, maintenance, and back-end infrastructure.
The deployment of the global positioning system (GPS) and the subsequent commercialization of GPS services have made GPS receivers a widely available consumer electronic device. GPS operates on the principle of multi-lateration calculation from the positions of four or more GPS satellites. The typical accuracy of position measured using GPS is about 15 m (50 ft) for consumer GPS receivers. The inherent limitations of GPS technology are that one is unable to obtain reliable positions within heavily built-up urban centers and the inability of obtaining any position measurement at all inside enclosed spaces such as within buildings. The latter has provided the impetus for indoor positioning systems (IPS) not only to complement GPS but also to enable a broad range of new location-aware applications (for example, smart rooms and offices).
An important incentive for the deployment of location services in mobile electronic devices is the U.S. Federal Communications Commission (FCC) requirement that cellular telephones support enhanced 911 (E911) services. Cellular service providers are required to determine the position of a 911 call from mobile telephones to enable emergency response. The accuracy of current mobile telephone location technology is typically 50-300 meters. The foregoing accuracy has limited utility for emergency response, particularly in the urban environment. Analysts estimate that 50% of cell phones will employ location services by 2010. The deployment of rudimentary location services on cell phones has enabled the creation of social networking services, for example.
The inability of cellular telephone service providers to accurately locate a given mobile telephone has led some to suggest the development of hybrid devices where cell phones also include a GPS receiver. Such a device enables improved location determination only in those areas where GPS is functional, GPS being limited once again in urban centers and inside buildings. A widely deployed IPS would be a candidate technology for being included within hybrid cell phones. It would be desirable for IPS technologies to seamlessly integrate with GPS technology to facilitate the development of hybrid mobile devices.
Another incentive for deploying accurate IPS is “smart room” technology. The deployment of location aware services in high rise buildings, for example, can allow fast and efficient evacuation of residents during fire alarms, provide services (such as indoor navigation) and enhanced quality of life for the mentally disabled, and provide emergency responders with efficient access to information regarding available entrances and exits and provide life-saving navigation and orientation services in zero-visibility environments (a major cause of firefighter deaths is disorientation).
Enterprise resource planning (ERP) systems support some or all of the basic functions of an organization, such as manufacturing, supply chain management, financials, projects, human resources, customer relationship management, decision support and data warehousing. In the retail sector, supply chain management is a key application, with retailers employing increasingly intelligent and location-aware products to manage inventories, suppliers and customers. Typically, an ERP system is underpinned by a single database that manages a vast array of data. Mobile electronic devices are increasingly used to track products at all points in the supply chain and a range of technologies are now employed in retail stores (e.g. point of sale devices) and distribution warehouses (e.g. RFID). Another embodiment of the invention herein could support an ultrasonic frequency identification (UFID) system.
Location systems known in the art include position determination using acoustic pulses underwater and long-baseline technologies in air. Very short baseline interferometry (VSBI) is part of the known art of sonar and radar signal processing (see for example, “Underwater Acoustic Positioning Systems” by P. H. Milne). A number of references describe the art of navigation and/or position determination using acoustics in water. U.S. Pat. No. 4,601,025 issued to Lea describes an apparatus for determining the angle of incidence of a received signal using two widely-spaced interferometers with equal-length crossing baselines. The Lea '025 patent points out that interferometric methods have long been in use in the sonar and radar art. The drawback of this invention is the use of “long baseline” interferometry for marine (or submarine) applications. Although high accuracy in position is possible with longer baselines, this feature is undesirable for in-building, in-air devices.
U.S. Pat. No. 4,800,541 issued to Farmer et al. describes a method of determining the bearing angle of a transmitted signal relative to a pair of remote signal receivers separated by several wavelengths of the transmitted pulse. The bearing angle is calculated as a function of the difference in phase of the transmitted pulse received at each receiver. Although the described invention uses the phase difference to calculate a relative bearing angle, the described invention does not purport to measure the range between the transmitter and receiver nor to calculate either absolute range and bearing to determine absolute position. The bearing angle is only calculated in the plane of the receivers and is defined relative to the axis between the receivers. The method does not measure or deliver absolute position in three dimensions as does our invention.
U.S. Pat. No. 5,615,175 issued to Carter et al. also describes a method for determining the bearing angle of an object located at a distance from a reference point. The object emits signals and a method is described by which the bearing angle is calculated using signal energy information rather than time-delay information. Again, this approach cannot provide an absolute measure of position and is not the objective of their approach. The accuracy of our invention is dependent on the multiplicity of acoustic sensors used to receive the signal and this is part of our uniqueness.
U.S. Pat. No. 5,659,520 issued to Watson et al. provides real-time estimation of the positions of multiple cooperative targets and a larger preferred baseline length to improve the time-delay estimates obtained using the known art of VSBI. The receiver array includes a transmitter to “request” a return pulse from the targets. This system uses the round-trip time-difference-of-arrival from the multiple targets to resolve the phase ambiguities arising from the longer baseline. The longer baseline system then has 10 to 20 times the accuracy of the very short baseline navigation technique. The advances to the known art of VSBI described in the Watson et al. patent are not applicable or of value to use in IPS devices. In the context of the present invention, it is only one transmitter target whose absolute position is being determined by the base station (the absolute position of the receiver array is known.
U.S. Pat. No. 6,141,293 issued to Amorai-Moriya et al. describes a method for ultrasonic positioning and tracking. The method depends on a multiplicity of ultrasound transmitters attached to the object being tracked and a multiplicity of background receivers placed at known positions around the tracking (or surveillance) volume. The range between the transmitters and background receivers are processed using prior art referenced therein from the time-of-flight data that is obtained. Position and orientation information is then deduced. A limitation of the device shown in the '293 patent is that the background receivers must be scattered around the volume of surveillance. The disclosed position algorithm employs range information exclusively.
U.S. Pat. No. 6,176,837 issued to Foxlin describes a system for tracking the position and orientation of a body. The '837 patent describes the use of inertial motion tracking of linear acceleration and angular velocity, which are integrated to determine linear velocity, linear displacement, and total angles of rotation. Accumulated drift in the measured values is corrected using methods described therein.
U.S. Pat. No. 7,283,423 issued to Holm et al. describes a method and system for detection, identification and position determination of chips which transmit ultrasound signals in a room. The system is particularly designed for the hospital environment where the emission of RF waves must be stringently controlled. A feature of the disclosed device is a method for correcting for the Doppler shift of signals when the chips are in motion. The system disclosed in the '423 patent does not purport to provide position measurements of the chips with high accuracy; it is deemed sufficient for the most part to localize the devices to within a given room in a building. When positioning to higher accuracy is required a multiplicity of receivers is required and a position is determined by complicated range calculations.
There continues to be a need for accurate position determination of mobile electronic devices within the confines of an enclosed space such as a building.